A (for example footway) barrier=kerb is a barrier for vehicles and wheelchair drivers. The height of the kerb is important and with this information, the usage by different groups can be determined. The height of the kerb is tagged additionally as height=*, if available. Right side is bottom, left side is top.

See also kerb=*, which is used on the node of a way at the location of the kerb.

Usage

which gives

Rendering

Renderers and editors might indicate the lower side of the kerb (the right side of the OSM way) with small triangles pointing downwards.

This shows what the arrows mean. The arrows point towards the lower ground.